The present invention relates to a system for controlling the ignition timing of an internal combustion engine having a turbocharger.
In an engine having a turbocharger, the pressure of the intake air is greatly increased by supercharging which results in an increase of the pressure of the mixture compressed in cylinders. Such a high supercharging pressure is apt to induce "knocking" in the engine.
Japanese patent laid open No. 56-990066 discloses a system for preventing this knocking. The system is provided with a signal port in the carburetor of the engine adjacent to the throttle valve and with a vacuum advance device, and it is adapted to control the ignition timing in the retard direction during supercharging operation and to control the timing in the advance direction at non-supercharging operation.
FIG. 1B shows the torque characteristic for different pressures at the signal port dependent on engine speed. When the pressure is higher than a positive pressure curve a, the ignition timing is changed in the retard direction, and when the pressure is lower than a negative pressure curve b, the ignition timing is shifted in the advance direction. FIG. 1A shows the ignition timing characteristic. As shown in FIGS. 1A and 1B, there is an insensitive zone between a and b. In other words, the characteristic is not continuous between peaks. Further, in the prior art, the amount of retarding of the ignition is determind only by the supercharging pressure without consideration of a factor relative to engine speed.
FIGS. 2A and 2B show supercharging characteristics from a point of view of engine characteristic, where FIG. 2A shows the characteristic of a low speed type turbocharger and FIG. 2B shows that of a high speed type turbocharger. In the engine having the low speed type turbocharger, it is not necessary to greatly change the ignition timing in the retard direction at a high engine speed zone, since supercharging efficiency decreases compared with a low engine speed zone. On the contrary, the ignition timing of the engine with the high speed type turbocharger should be retarded at a high engine speed zone because of high supercharging efficiency. These facts mean that the ignition timing should be controlled by pressure relative to engine speed in addition to supercharging pressure.